RESEARCH COMMUNICATIONS are used for food, medicine, fertilizer, etc.13. Therefore, it the Australian Institute of Marine Science, Australia, 1998, p. is a cause of concern that coral reefs and associated or- 184. ganisms are threatened by the El Niño Southern Oscilla- 11. Hyne, J., Bleaching, the great unknown. Reef Management News, 15,16 Reef Research, June 1998, vol. 8, pp. 8–11. tion which affects the global climate . This in turn, 12. Coral bleaching and global climate change – current state of know- causes fluctuations in rainfall, resulting in drought in ledge, CRC Reef Research Centre, Townsville, Australia, January some areas and heavy rainfall in others17. Guerrero17 had 2002. discussed the impacts of El Niño on Philippine fisheries. 13. Birkeland, C. (ed.), Life and Death of Coral Reefs, Chapman & He had reported that for marine fisheries, the fishermen Hall, New York, USA, 1997, p. 536. 14. Salvat, B. (ed.), Coral Reefs – A Challenging Ecosystem for Hu- operating in near-shore shallow coastal waters had lower man Societies, Global Environmental Change, 1992, vol. 2, pp. catch per unit effort (CPUE), while commercial fisher- 12–18. men operating in deep waters had relatively high CPUE 15. Glynn, P. W., Widespread coral mortality and the 1982–1983 El during El Niño. Kumaraguru8 had also reported a reduc- Niño warming event. Environ. Conserv., 1984, 11, 133–146. tion in distribution of coral-associated fishes due to blea- 16. National Oceanic and Atmospheric Administration, What is El Niño-Southern Oscillation (ENSO)? US Department of Com- ching in the reefs of Palk Bay and Gulf of Mannar regions. merce, 1997, p. 6. This could be also due to an apparent movement of small 17. Guerrero, R. D., The impacts of El Niño on Philippine Fisheries, pelagic fishes from the shallow to the deeper waters18. Naga, The ICLARM, 1999, vol. 22, pp. 14–15. Although the corals have got bleached, they need to be 18. Armada, N., An elevation of the effect of El Niño on the small pe- protected because these bleached reefs will slowly recover lagic fishes of the Visayan Sea. National workshop on the assess- ment of the impacts of El Niño on Philippine Fisheries, Los Banos, in time by way of new growth of coral colonies over the Laguna, Philippines, 20–21 August 1998, p. 10. old ones. The optimistic point to note here is that if we look at the scientific records, this phenomenon of coral ACKNOWLEDGEMENTS. We thank the Global Coral Reef Moni- bleaching has been taking place periodically all over the toring Network of the IOC/UNESCO, the Ministry of Environment and world. The process of natural selection is in operation, with Forests, Govt. of India, and the Department of Ocean Development, Govt. of India for grants provided to carry out coral reef research work the growth of new coral colonies and any disturbance in in the Gulf of Mannar and Palk Bay regions in the southeast coast of the system is only temporary. Therefore, in spite of the India during the period 1998 to 2002. We thank Mr P. Thavasi, odds, the corals will resurge under the sea, which we SCUBA diver for technical assistance in the field. need to protect and conserve for our benefit. Received 9 December 2002; revised accepted 19 September 2003

1. Nybakken, J. W., Marine Biology – An Ecological Approach, Harper and Row, New York, 1988, 2nd edn, p. 514. 2. Gopinadha Pillai, C. S., Composition of the coral fauna of the south- Preliminary observations from a eastern coast of India and the Laccadives. Proceedings of the Sym- trench near Chandigarh, NW posium of the Zoological Society, London (eds Stoddart, D. R. and Murice Younge), Academic Press, 1971, vol. 28, pp. 301–327. Himalaya and their bearing on active 3. English, S., Wilkinson, C. and Baker, V. (eds), Survey Manual for Tropical Resources, Australian Institute of Marine Sciences, faulting Townsville, Australia, 1997, 2nd edn, p. 390. 4. Strickland, J. D. H. and Parsons, T. R., A Practical Handbook of Javed N. Malik†,*, Takashi Nakata#, Seawater Analysis, Bulletin 167, Fisheries Research Board of ‡ ‡ Canada, Ottawa, 1977, 2nd edn, p. 310. George Philip and N. S. Virdi 5. Too much stress for the reef? CRC Reef News, CRC Reef Res- †Department of Civil Engineering, Indian Institute of Technology, earch Centre, Townsville, Australia, June 2002, vol. 9, p. 3. Kanpur 208 016, India 6. Lally, K. and Berkelmans, R., Coral bleaching and climate change #Department of Geography, Hiroshima University, Higashi, on the Great Barrier Reef – an update, CRC Reef Research Cen- Hiroshima 739, Japan ‡ tre, Townsville, Australia. June–September 1999, pp. 4–5. Wadia Institute of Himalayan Geology, Dehra Dun 248 001, India 7. Kumaraguru, A. K., Kannan, R. and Sundaramahalingam, A., Re- port on monitoring coral reef environment of Gulf of Mannar – A Evidence of two parallel to sub-parallel active-fault pilot study. Report submitted to IOC/UNESCO, Paris, France traces along the Himalayan Front around Chandigarh through Global Coral Reef Monitoring Network South Asia, Sri has provided additional information on the imbri- Lanka, 1999. cated faulting pattern that branches out from the 8. Kumaraguru, A. K., Assessment of impact of bleaching phenome- Himalayan Frontal Thrust system, which probably non on the corals and their recovery in the Gulf of Mannar and Palk Bay, Ministry of Environment and Forests, Government of merges down northward with décollement. Displace- India, New Delhi, 2002. ment in terraces along these branching faults and the 9. Wilkinson, C. R., Linden, O., Cesar, H., Hodgson, G., Rubens, J. maximum height of the scarp (38 m) suggest contin- and Strong, A. E., Ecological and socioeconomic impacts of 1998 ued tectonic movement since late Pleistocene and cumu- coral mortality in the Indian Ocean: An ENSO impact and a warn- lative slip along the faults. The preliminary trench ing of future change. Ambio, 1999, 28, 188–196. 10. Wilkinson, C. R. (ed.), Status of Coral Reefs of the World, Pub- lished on behalf of the Global Coral Reef Monitoring Network by *For correspondence. (e-mail: [email protected])

CURRENT SCIENCE, VOL. 85, NO. 12, 25 DECEMBER 2003 1793 RESEARCH COMMUNICATIONS investigation across the young fault reveals a total dis- quakes during this century4. Studies carried out during placement of 3.5 m along a thrust fault, indicating the past several decades have provided important data on remnant of one large-magnitude prehistoric earth- the ongoing crustal deformation in the Himalaya1,5–7. quake. However, not much effort is spent in site-specific studies.

Here we present some preliminary paleoseismic observa- IN the Himalaya, instrumental records suggest that the tions from trench excavated near Chandigarh (Figure major seismic activity with moderate-magnitude earth- 1 b). quakes (> 6 or < 7) is concentrated to an approximately The study area around Chandigarh marks the south- 50 km-wide belt located in the Lesser Himalaya, south of ernmost fringe of the Himalaya, where the undeformed the Main Central Thrust (MCT)1. In a span of the last succession of the Indo-Gangetic Plains is separated from 100–120 years, four large-magnitude events have occurred the detached, complex folded-faulted Upper Siwalik Hills along the Himalayan foothill zone: the 1897 Shillong comprising molassic sediments of lower Pliocene–early (M 8.7), 1905 Kangra (Ms 7.8)2, 1934 Bihar (M 8.4), and Pleistocene age. The boundary is well-defined by the the 1950 Upper Assam (M 8.5) earthquakes (Figure 1 a). Himalayan Frontal Thrust (HFT) system (Figure 1 b). None of these produced any surfaces rupture. The area The Ghaggar river, which is the major drainage in this between 1905 Kangra and 1934 Bihar–Nepal earthquakes area flows SSW, cuts transversely the NNW trending is categorized as the Central Seismic Gap3, which bears Upper Siwalik Hills before debouching into the plains. high probability for one or more M > 8 Himalayan earth- Five terrace levels were observed along the right bank

a

b

Figure 1. a, Map showing meizoseismal zones of four great earthquakes along the Himalayan Front and other large-magnitude events (after Yeats and Thakur15); b, Generalized geological map of north- western Himalayan Front around Chandigarh (after Gansser16 and Valdiya17). (Inset) DEM of India show- ing location of Chandigarh fault.

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Figure 2. Distribution of terraces and active fault traces along northwestern Himalayan Foot Hill Zone around Chandi- garh along the right bank of Ghaggar river. Box highlights the area of satellite photograph and location of trench site. CF, Chandigarh Fault; HFT, Himalayan Frontal Fault (modified after Nakata8).

east of Chandigarh8. The remote-sensing data and field studies have shown existence of active fault traces in the foothill zone, with the average length varying from 2 to 10 km and strike from N335° to N15°E (Figures 2 and 3). The digression in the strike is due to the sinuous nature of the mountain-front. The surface expressions of the faults are marked by discontinuous pattern. It is assumed that the fault connects subsurface, but has not ruptured up to the surface at all places. Besides, there occur straight, linear mountain-fronts marked by well-developed triangu- lar facets indicating the ongoing tectonic activity in this region, because tectonically-active fronts usually display prominent, large facets that are generated along the base of the escarpments9,10. Figure 3. Stereo-pair of satellite photographs showing active fault Two traces of active faults running parallel to one an- traces (marked by red arrows) along the Himalayan Front, southeast of Chandigarh on the right bank of Ghaggar river (refer Figure 2 for loca- other between Panchkula and Mansadevi with strikes tion). Location of trench is shown by yellow arrow. N335° to N15°E have vertically displaced and warped

the Ghaggar, Kalka, Pinjore and Koshallia terraces com- where it crosses the Upper Siwalik Hills (Figures 1 b and prising late Pleistocene to Holocene alluvial-fan sediment 2). From the amount of dissection and level of strati- succession (Figures 2 and 3). Vertical uplift along these graphic position, these terraces/surfaces are categorized faults has resulted into 15 to 38 m high fault scarplets as the Ghaggar, Kalka, Pinjore, Koshallia and younger facing the Gangetic Plains. These scarps are transverse to alluvial terraces5. It is presumed that these terraces might the course of the Ghaggar river channel. The distance bet- have developed due to incision caused by periodic uplift ween the faults varies from 0.25 to 0.3 km. The first fault along the Himalayan Front. The surface morphology and trace exhibits a 20 m high fault scarp along its northwest sediment content are dominated by matrix-supported gra- fringe, which was responsible for uplifting the Kalka and velly clasts (pebble–boulder) of sandstones derived from Pinjore terraces near Mansadevi, and the scarp height re- Lower Siwalik Hills in the upper reaches; these terraces duces up to 6 m towards southeast where it has displaced are co-relatable with the terraces occurring in the Pinjore the Koshallia terrace (Figures 2 and 3). This fault is named Dun valley5. as the ‘Chandigarh fault’ (CF). We carried out trenching to The most prominent deformation along the HFT was confirm the amount of displacement and the pattern of observed on the right bank of the Ghaggar river, north- faulting of the Chandigarh fault system (Figures 2 and 3).

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Figure 4. Northwall view of E–W trending trench at the base of Chandigarh fault scarp near Mansadevi (refer to Figures 2 and 3 for location). Fault strands are marked by red arrows. Thread on the wall shows grid of 1 m ´ 1 m. Cd, Channel deposit; S1 and S2, Massive sand deposits; Cb, Channel bar; Ocd, Older channel deposit.

Figure 5. Detail trench log of northwall. Line along the fault trace shows the amount of displacement measured between the displaced Cd and S1 units taking into consideration the lower bounding surface between them.

At the base of a N15°E striking 20 m high fault scarp This phase of deposition was responsible for the degrada- along CF, a 14 m long, 2 to 4 m deep and 4.5 m wide E– tion of the scarp and erosion of the channel-fill deposit. W trending trench was excavated (Figures 2 and 3). The The fault plane was traced for about 6.5 m from the base trench was subsequently logged and documented (Figures in the east up to near-surface in the west. The fault shows 4 and 5). Exposed succession in the trench was catego- variable dip ranging from 20 to 46° due east. The fault is rized into unit Ocd–older channel fill deposits composed marked by a steeper dip of about 46° near the surface in of loose matrix (very fine gravel + medium–coarse sand + the western portion of the trench, and reduces to about silt)-supported pebble–cobble–boulder of sandstone of 20° towards the east. In the central segment the dip of the Dagshai, Kasauli and Subathu Formation, units S1 and fault is as low as 4 to 8°, and again becomes steeper in S2 – massive medium-to-coarse sand facies, unit Cd – the lower part. The fault plane is distinctly marked by 10 channel-fill deposit consisting pebble–cobble and unit to 12 cm thick crushed zone or brecciated zone compris- Cb – channel bar made of comparatively finer gravel ing angular gravel, where it passes through the S1 unit clasts seen in Ocd and Cd facies. (Figures 4 and 5). The brecciated zone suggests that the A fault strand (F) has displaced Ocd, S1 and Cd litho- gravel clasts were crushed and dragged along the fault units in the eastern and central portion of the trench (Fig- during the movement from the underlying lithounit. Fur- ures 4 and 5). The external geometry of unit Cd shows ther west, the fault contact between the S1 and Cd units slightly erosive contact with respect to the underlying S1 shows typical shear-fabric marked by aligned discoidal unit and concave-up lower-bounding surface representing pebbles along their long axes orientated parallel to the shallow channel-fill deposits. Overall succession is capped fault-plane contact (Figures 4 and 5). Total displacement by slope deposit, which was deposited after the event. of about 3.5 m was measured taking into account the

1796 CURRENT SCIENCE, VOL. 85, NO. 12, 25 DECEMBER 2003 RESEARCH COMMUNICATIONS lower bounding surface between Cd and S1 units. The seismic event/s. However, the extensively-occurring Pin- movement from east to west has resulted into slight de- jore surface does not show well-developed reddish soil formation within unit Cd, which resembles folding near cover. On the basis of this, Nakata8 suggested that the the tip of the fault. Since we did not find any distinct sig- Pinjore surface probably formed during Last Glacial natures of pedogenic features in the sandy lithounit (S1), period. The OSL ages of the Quaternary deposits from we presume that unit S1 is quite young, probably repre- the Pinjore Dun indicate that the sedimentation commen- senting late Holocene surface, which was subsequently ced in this valley well before 57 ka BP and continued up to overlain by unit Cd and finally displaced along the thrust around 20 ka BP12. If this is acceptable, then the Koshallia fault. and other younger surfaces might have developed bet- The occurrence of two parallel to sub-parallel active ween upper Pleistocene to Holocene times, and since then fault traces probably represents the branching-out faults these surfaces have been subsequently subjected to perio- of the HFT system that probably merge down northward dic/episodic uplift and deformation. The gentle warping with décollement; their geometry typically resembles the of the terraces near the fault line on the hanging wall as imbricated faulting pattern seen in the fold and thrust well as warping in the sediment succession suggest that belt11. Displacement of the Ghaggar, Kalka, Pinjore and this deformation is due to the movement over the bend in Koshallia terraces along CF and the maximum height of the fault with a steeper fault plane near the surface. This the scarp (38 m) suggest continued tectonic movement type of geometry of deformation is commonly associated since late Pleistocene and cumulative slip along the fault. with the thrusts that normally step-up in the direction of However, we did not observe any re-orientation of gravel slip to a higher décollement or ramp in décollement, as clasts or evidence of repeated events; thus it is suggested defined by Suppe13. that the faulting we observed in the trench represents the The displacement of the youngest unit Cd in the trench, latest event. Owing to lack of age constraints of the res- and insignificant cover of well-developed soil over it pective terraces, it is difficult to estimate the timing of the suggest that this is the latest event in this area. The net displacement along the fault (F) of about 3.5 m indicates a single large-magnitude paleoearthquake event (Figure 6). Taking average angle of the fault (25°) gives vertical displacement of about 1.5 m and horizontal crustal short- ening of about 3.2 m for one large magnitude event. In addition to this recent paleoseismic investigation of Black Mango Tear Fault, a tear fault along the HFT in northwestern Himalaya has revealed evidence of two large surface-rupture earthquakes during the past 650 years, subsequent to AD 1294 and AD 1423 and another at about AD 260 (ref. 14). With the assumption that HFT dips 30 ± 10°, a fault slip and crustal shortening of +7.0 +7.0 9.62-3.5 mm/year and 8.42-3.6 mm/year respectively was proposed. This suggests that the HFT system and its branching-out faults have been ruptured in the historic past, and are active faults which are capable of triggering large magnitude earthquakes in future. The active faults identified, here in addition to that by Nakata8 and the preliminary paleoseismic investigation have provided significant information towards the ongo- ing tectonic deformation along the Himalayan arc. Paleo- seismic studies are essential to construct a long-term earthquake database of unknown large magnitude prehis- toric events, which will help in evaluating the seismic hazard of these regions. Figure 6. Stage I: Simplified version of trench log showing strati- graphic position of lithounits before faulting, after eliminating the dis- placement along the fault. Cd represents the channel trough and marks 1. Valdiya, K. S., The Main Boundary Thrust zone of Himalaya, erosive contact with the underlying S1 (sandy) unit and other units like India. Ann. Tectonicae, 1992, 6, 54–84. S2 and Cb on the left-hand-side of the succession. Stage II: Trench log 2. Ambraseys, N. and Bilham, R., A note on the Kangra Ms = 7.8 showing stratigraphic relationship of the lithounits after faulting, where Cd and S1 have been faulted and moved up. OCd unit must have been earthquake of 4 April 1905. Curr. Sci., 2000, 79, 45–50. upthrusted from below. Stage III: Final picture of the trench after the 3. Khattri, K. N. and Tyagi, A. K., Seismicity patterns in the Hima- scarp was subjected to erosion, followed by phase of deposition of layan plate boundary and identification of areas of high seismic slope-derived material. potential. Tectonophysics, 1983, 96, 281–297.

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4. Bilham, R., Gaur, V. K. and Molnar, P., Himalayan seismic haz- 14. Kumar, S., Wesnousky, S. G., Rockwell, T. K., Ragnon, D., Tha- ard. Science, 2001, 293, 1442–1444. kur, V. C. and Seitz, G. G., Earthquake recurrence and rupture 5. Nakata, T., Geomorphic History and Crustal Movements of Foot- dynamics of Himalayan Frontal Thrust, India. Science, 2001, 294, hills of the Himalaya, Sendai, Institute of Geography, Tohuku 2328–2331. University, 1972, p. 77. 15. Yeats, R. S. and Thakur, V. C., Reassessment of earthquake haz- 6. Yeats, R. S., Nakata, T., Farah, A., Fort, M., Mirza, M. A., ard based on a fault-bend fold model of the Himalayan plate- Pandey, M. R. and Stein, R. S., The Himalayan frontal fault boundary fault. Curr. Sci., 1998, 74, 230–233. system. Ann. Tectonicae, 1992, 6, 85–98. 16. Gansser, A., The Geology of the Himalaya, Wiley Interscience, 7. Wesnousky, S. G., Kumar, S., Mohindra, R. and Thakur, V. C., New York, 1964, p. 189. Uplift and convergence along the Himalayan Frontal Thrust. Tec- 17. Valdiya, K. S., Aspects of Tectonics, Focus on South-Central tonics, 1999, 18, 967–976. Asia, Tata McGraw-Hill, New Delhi, 1984, p. 319. 8. Nakata, T., Active faults of the Himalaya of India and Nepal. Geol. Soc. Am., Spl. Pap., 1989, 232, 243–264. 9. Bull, W. B., Tectonic geomorphology. J. Geol. Educ., 1984, 32, ACKNOWLEDGEMENTS. We are grateful to referees for their con- 310–324. structive and valuable comments. Financial support provided to J.N.M. 10. Wells, S. G. et al., Regional variations in tectonic geomorphology and T.N. by Japan Society for Promotion of Science (JSPS), Tokyo, is along a segmented convergent plate boundary, Pacific coast of acknowledged. J.N.M. is grateful to JSPS for providing him post- Costa Rica. Geomorphology, 1988, 1, 239–265. doctoral fellowship for period 1999–2001, to undertake this work in 11. Burbank, D. W., Verges, J., Munoz, J. A. and Bentham, P., Coe- Japan at Department of Geography, Hiroshima University. J.N.M. also val hindward- and forward-imbricating thrusting in the south- thanks DST for financial support (vide project no. SR/FTP/ central Pyrenees, Spain: Timing and rates of shortening and deposi- ES/46/200). Permission provided by Dr Samra, Dy. Director General tion. Geol. Soc. Am. Bull., 1992, 104, 3–17. (NRM), ICAR, New Delhi and Dr V. N. Sharda for trench excavation 12. Suresh, N., Bagati, T. N., Thakur, V. C., Kumar, R. and Sangode, at CSCR farm, Mansadevi, and help by Dr Aggarwal, Head, CSCR, S. J., Optically simulated luminescence dating of alluvial fan depo- Chandigarh are acknowledged. sits of Pinjaur Dun, NW Sub-Himalaya. Curr. Sci., 2002, 82, 1267–1274. 13. Suppe, J., Geometry and kinematics of fault-bend folding. Am. J. Sci., 1983, 283, 684–721. Received 26 December 2002; revised accepted 28 August 2003

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